1. Field of the Invention
The present invention relates generally to implantable cardioverter-defibrillator (ICD) systems, and more particularly, to a prophylactic or preventive ICD system.
2. Background Information
Little effort has been expended in developing ICDs for people having only mildly abnormal cardiac conditions. Virtually all existing ICD systems are designed for people with severely abnormal hearts, and hence, these ICD systems have been used for lifesaving intervention in crisis situations. Such ICD systems, for a substantial number of reasons, are unsuited for use by patients whose need is for less severe treatment, and for protective intervention at most.
One factor which distinguishes the use of ICD systems as a treatment therapy, rather than a prophylactic therapy, relates to the criteria embodied in the ICD system that determine when an electrical shock should be administered. Although substantial progress is being made currently in devising and developing more accurate methods for identifying tachycardias and fibrillations, uncertainties remain. Hence, it is necessary to provide a detection "guard band" or to acknowledge a range of uncertainty in relation to the appropriateness of shock delivery. When the ICD system is used as treatment therapy for immediate lifesaving purposes, the criterion used to determine when an electrical shock should be delivered can lead to the delivery of a number of "false-positive" shocks, even when the patient does not necessarily require shock therapy. A false-positive shock administration is painful and disconcerting to the patient, and potentially hazardous as well, however, delivery of a false-positive shock is preferred to the alternative of overlooking a true cardiac crisis that might not otherwise be detected due to the uncertainty of existing detection criterion.
Other factors which distinguish the use of an ICD system as a treatment therapy for an existing severe cardiac condition, rather than as a prophylactic therapy for a prospective cardiac incident, involve the number of shocks that the ICD system must be designed to be able to deliver during its implanted life and the energy of these delivered shocks.
All of the existing ICD systems which have been approved by the Federal Drug Administration (FDA) require that the ICD system be capable of delivering at least 250 shocks and up to 400 shocks, each shock having a maximum stored energy value of between 34 to 40 joules. Together with the energy required for cardiac monitoring over the lifetime of the device, these requirements dictate that the ICD system must have initial stored energy requirements of at least 15,000 to 20,000 joules or more. Both of these factors directly affect the physical size of the ICD system due to the dominance of the battery and the discharge capacitor in determining the physical volume of the ICD system package. The size of the ICD system, in turn, affects possible implantation sites. As a result, all of the existing ICD systems approved by the FDA are larger than about 120 cc and require implantation in the relatively spacious abdominal cavity. In order to obtain effective current distribution for a defibrillation shock when implanted in the abdominal cavity, existing ICD systems often must use patch electrodes that require transthoracic implantation. In contrast, implantable pacemakers, for example, are much smaller in volume and can be preferably implanted in the pectoral region and are connected to the heart by transvenous catheters, thereby simplifying the implantation procedure. Not surprisingly, the larger ICD systems are also more costly, and this too, limits their utility in prophylactic applications.
A number of studies have been initiated to determine the viability of using existing ICD systems for prophylactic purposes. Nisam, S. et al. "Identifying patients for prophylactic automatic implantable cardioverter defibrillator therapy: Status of prospective studies", Am Heart J, 1991; 122:607-612; MADIT Executive Committee. "Multicenter Automatic Defibrillator Implantation Trial (MADIT): Design and Clinical Protocol", PACE, 1991; 14:920-927; Moss, A., "Prospective Antiarrhythmia Studies Assessing Prophylactic Pharmacological and Device Therapy in High Risk Coronary Patients", PACE, 1992, 15:694-96; and Kuck, K. et al, "Prospective Studies Assessing Prophylactic Therapy in High Risk Patients: The German Dilated CardioMyopathy Study (GDCMS)--Study Design", PACE, 1992, 15:697-700. In all of these studies, however, existing ICD systems are being used and there are no suggestions to how to design a prophylactic ICD system.
The need for a prophylactic implantable cardioverter defibrillator has long been recognized by the medical community, but there presently is no agreement on how such a device should be designed. While many of the issues might appear at first to be relatively simple design choices, cardiac experts cannot decide on how a prophylactic ICD system should be implanted, what therapies it should offer, how much the device should cost, or how many shocks the device will need to be able to deliver. Hauser, R., "Attributes of a Prophylactic Implantable Cardioverter Defibrillator: How Close Are We?", PACE, Vol. 16 (Mar. 1993), pp. 582-85.
In his article, for example, Dr. Hauser directly addresses the issue of the number of shocks required of a prophylactic ICD device. "Another question is the number of shocks a PAICD [prophylactic automatic implantable cardioverter defibrillator] should be capable of delivering. Experts cannot agree. The short-term goal is to rescue the patient from the initial episode and to provide defibrillation back-up until the patient can be transported to the appropriate medical facility. Are 5 or 10 or 50 shocks sufficient to accomplish this goal?. Further, what do you do with the patient after the initial event?. Replace the unit with a standard ICD?. This strategy adds not only to cost, but also to potential patient morbidity (e.g., infection related to the replacement procedure). These are important questions whose answers will profoundly affect the economics of prophylactic devices and how the medical community and society view this therapy." Id. at 583-84.
While existing ICD systems have proven effective when used as a treatment therapy for severe cardiac conditions, these devices are not adapted to meet the recognized needs of an ICD that would provide prophylactic therapy for a prospective cardiac incident in patient's with less severe cardiac conditions. In addition, while some of the goals of a prophylactic ICD systems are well known, there is simply no agreement in the medical community on how, or even if, these goals can be achieved. Accordingly, it would be desirable to provide a design for a practical prophylactic ICD system that can meet the ultimate goal of providing effective prophylactic therapy for prospective cardiac incidents in patients with less severe cardiac conditions.